The field of the invention is isobutene and the present invention is particularly concerned with the production of high-purity isobutene by dehydrating tertiary butanol (TBA) in the presence of a strongly acidic synthetic resin cation exchanger as the catalyst.
The state of the art of processes for producing isobutene by dehydrating TBA may be ascertained by reference to U.S. Pat. Nos. 2,377,026; 3,510,538; 3,665,048; 4,012,456; 4,036,905; and 4,155,945, and British Pat. No. 2,022,129, the disclosures of which are incorporated herein.
Ion exchange resins suitable in the present invention are disclosed in U.S. Pat. Nos. 2,480,940; 2,922,822; and 3,256,250, and British Pat. No. 957,000, the disclosures of which are incorporated herein.
TBA dehydration takes place reversibly and endothermally. The conversion into isobutene and water increases with rising temperature in the view of the chemical equilibrium.
Accordingly, in industry the TBA dehydration is predominantly carried out at high temperatures of about 180.degree. to 450.degree. C. in the presence of weakly acidic catalysts such as silica gel, thorium oxide, titanium(IV)-oxide or especially aluminum oxide as disclosed in U.S. Pat. Nos. 4,036,905; 3,665,048; and 2,377,026. While a high TBA conversion is achieved using high temperatures, the considerable losses in isobutene by its oligomerization on the other hand represent a drawback. In addition to isobutene oligomerization there also takes place an undesired isomerization of the formed isobutene to n-butenes as disclosed in U.S. Pat. Nos. 3,665,048, and 2,377,026, causing further product losses, more significantly however making the separation and extraction of pure isobutene especially disadvantageously difficult.
When the dehydration of TBA is carried out at lower temperatures (below 180.degree. C.) more active and strongly acidic catalysts are needed. Due to their inherent drawbacks such as corrosion, waste water problems, costly recovery, etc., the homogeneous acid catalysts such as mineral acids, organic sulfonic acids and heteropoly acids drop in significance for the dehydration reaction. On the other hand, cation exchanger resins assume significance as catalysts.
Due to the poorer equilibrium conditions, only partial TBA conversions remain possible at lower temperatures. Therefore higher degrees of conversion require continuous removal of the reaction products.
This continuous removal is achieved by setting lower pressures of reaction below 2 bars, so that the isobutene distills off as a low boiling point component from the reaction mixture in continuous manner together with a partial-pressure determined by the proportion of TBA and water as disclosed in U.S. Pat. No. 3,256,250. As the rate of formation--which in any event is low--strongly drops as the water content in the reaction mixture increases, such procedures are only suitable for reaction mixture rich in TBA. Process steps have been described for TBA solutions which are richer in water, wherein inert entraining agents are proposed for the water of reaction as disclosed in U.S. Pat. Nos. 3,510,538 and 4,155,945. For instance when adding benzene or xylene as the entraining agent, there takes place however in the light of the teaching of U.S. Pat. No. 3,510,538 and especially above 100.degree. C. a reinforced oligomerization of isobutene. Moreover, the addition of a third substance supporting the reaction, on the other hand, hampers reprocessing because this accessory substance must be discharged again and be recovered.
It has been suggested in order to overcome these drawbacks that the aqueous TBA solution be fed to the rectifier part of a column from which, preferably at 85.degree. C. and a maximum pressure of 1.9 bars an aqueous vapor flow rich in TBA is driven through the catalytic bed mounted in loose bulk above the rectifier section as disclosed in U.S. Pat. No. 4,012,456. Together with the proportion of TBA and water, the isobutene is withdrawn in gaseous form from the reaction section. This method is satisfactory with regard to selectivity, but it does require a specially formed cation exchange resin not on the market.
In addition to the frequently complex reaction procedure, the prior art methods basically incur a substantial drawback:
Isobutene is obtained in gaseous form together with a partial-pressure determined proportion of TBA and water at a pressure no longer permitting it to condense using the conventional coolants used in industry.
The extraction of pure isobutene by rectification therefore requires costly compression and condensation stages. To avoid this drawback, the method of British Pat. No. 2,022,129 performs the dehydration of TBA in the presence of acid cation ion exchange resins at a pressure of at least 5 bars. The temperature is selected correspondingly to be so high that the produced isobutene can be removed in gaseous form together with the proportion of TBA and water from the reaction zone. Depending on the pressure of reaction, temperatures of about 180.degree. C. or higher are required in this procedure, that is, these are temperatures above the limit of application of commercial cation exchange resins and from which therefore only a restricted catalyst life can be expected under these conditions. Moreover the higher temperatures entail a lesser isobutene selectivity compared to the previously cited methods. There is a common factor in all these procedures reflecting the state of the art for TBA dehydration in the presence of acid synthetic resin exchangers, namely that the components of reaction are present in a mixed phase consisting of a gaseous and a liquid phase, and that the catalyst is present as a third and solid phase. Thereby the mass transfer to the catalytically effective centers of the catalyst will be hampered, and as a result, the rate of isobutene formation referred to the reactor volume, i.e., the time-space-yield, is unsatisfactory for isobutene.
Accordingly, large amounts of catalyst are required to implement the process on an industrial scale. This causes problems regarding the apparatus to be used and the heat supply to the strongly endothermal splitting reaction, as heat from only a heat source of comparatively low temperature can be fed to the synthetic exchangers being used on account of the low temperature stability, as a result of which a large heat transfer area is required.
To improve the mass transfer conditions for splitting, the methods of the state of the art use a movable catalytic bed. The synthetic resin structure of commercial cation exchangers when used in polar substances such as TBA and water, experiences marked swelling and deformation substantially degrading the mechanical stability of the catalyst.
Therefore, it is impossible to prevent abrasion and fracture of the catalyst particles in a moving catalytic bed.